Back to: Organic Chemistry 400 Level
Welcome to class!
Hello outstanding learner, it’s great to have you here once again. I hope you are feeling confident and ready, because today we will move further into the fascinating world of molecular analysis with Advanced Spectroscopy II. In the last lesson, we introduced the basic ideas behind spectroscopy and discussed UV-Vis and IR spectroscopy. In today’s class, we will focus on two powerful and highly detailed techniques — Nuclear Magnetic Resonance (NMR) Spectroscopy and Mass Spectrometry (MS), both of which are essential for structure determination in modern organic chemistry laboratories.
Advanced Spectroscopy II
Just think of NMR as a way of “interviewing” the atoms in a molecule to find out who they’re connected to, and mass spectrometry as a way of “weighing” the pieces of a molecule to find out what it’s made of. When used together, they give us a very clear picture of the entire molecule.
Nuclear Magnetic Resonance (NMR) Spectroscopy
NMR spectroscopy is a technique that analyses the magnetic properties of atomic nuclei (usually hydrogen or carbon). When placed in a strong magnetic field and exposed to radio waves, certain nuclei absorb energy and give signals. These signals depend on the chemical environment of each nucleus and provide information about the structure of the molecule.
Important features in ¹H NMR include:
Chemical Shift: This tells you the type of hydrogen present (e.g., aromatic, aliphatic or attached to oxygen). It is measured in parts per million (ppm).
Integration: Shows how many hydrogens contribute to a particular peak.
Splitting (Multiplicity): Peaks can be split into doublets, triplets or quartets depending on how many neighbouring hydrogens are present. This helps reveal which atoms are near each other.
A good way to imagine this is to think of your neighbourhood. Chemical shift tells you which street a person lives on, integration tells you how many people live in that house, and splitting tells you how many neighbours they interact with.
Mass Spectrometry (MS)
Mass spectrometry is used to determine the molecular weight and fragmentation pattern of a compound. In this technique, molecules are ionised in the gas phase and then separated based on their mass-to-charge ratio (m/z). The instrument produces a spectrum showing peaks for each fragment.
Key features include:
Molecular Ion Peak (M⁺): This is often the highest mass peak and corresponds to the intact molecule. From this peak, you get the molecular weight.
Fragmentation Peaks: Smaller peaks formed from the breakdown of the molecule. These help identify specific parts of the molecule.
For example, a peak at m/z 77 often indicates a benzene ring fragment. Just like seeing pieces of a broken calabash and telling which part came from the rim or the body, mass spectrometry tells you which fragment came from which section of the molecule.
Combination of NMR and MS
In practice, organic chemists rarely rely on just one technique. Combining NMR and MS gives a more complete picture:
NMR shows the arrangement of atoms and how they are connected.
MS gives the exact molecular weight and reveals structural fragments.
This is like using both a map and GPS while travelling — the map shows you the structure of the road network (NMR) and the GPS gives your precise location and direction (MS).
Summary
- NMR spectroscopy studies the magnetic behaviour of nuclei to determine molecular structure.
- Chemical shift, integration and splitting patterns in ¹H NMR provide detailed information about hydrogen atoms and their environments.
- Mass spectrometry determines the molecular weight and shows fragment patterns that identify parts of the molecule.
- The molecular ion peak (M⁺) indicates the molecular mass, while fragmentation peaks help identify structural fragments.
- Using NMR and MS together gives a complete and accurate understanding of an organic compound’s structure.
Evaluation
- What information does chemical shift provide in ¹H NMR spectroscopy?
- What is the molecular ion peak in mass spectrometry and what does it represent?
- Why are NMR and MS often used together in organic synthesis?
- Explain what peak splitting reveals in an NMR spectrum.
Well done for staying focused and engaged! You are doing exceptionally well. Keep up the amazing work — Afrilearn is proud to support you every step of the way, and the next lesson will deepen your knowledge even more!